Syrians run for cover as a helicopter hovers over the northern city of Aleppo on July 24, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

Syrians run for cover as a helicopter hovers over the northern city...

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Syrian rebels man a checkpoint in the northern city of Aleppo on July 24, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

Syrian rebels man a checkpoint in the northern city of Aleppo on...

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Syrians run for cover as a helicopter hovers over the northern city of Aleppo on July 24, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

Syrians run for cover as a helicopter hovers over the northern city...

Image 4 of 9

Syrian rebels man a checkpoint in the northern city of Aleppo on July 24, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

Syrian rebels man a checkpoint in the northern city of Aleppo on...

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Syrians walk past a checkpoint manned by rebels in the northern city of Aleppo on July 24, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

Syrians walk past a checkpoint manned by rebels in the northern...

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A Syrian civilian who was wounded during clashes between Syrian troops and rebels rests at a makeshift clinic in the northern city of Aleppo on July 24, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

A Syrian civilian who was wounded during clashes between Syrian...

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A Syrian rebel takes position as a helicopter hovers over the northern city of Aleppo on July 23, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Photo: Bulent Kilic, AFP/Getty Images

A Syrian rebel takes position as a helicopter hovers over the...

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A Syrian rebel stands guard in a street in the northern city of Aleppo on July 23, 2012. A commercial hub and home to 2.5 million people, Syria's second city Aleppo has become a new front in the country's 16-month uprising, after being largely excluded from the violence. AFP PHOTO / BULENT KILICBULENT KILIC/AFP/GettyImages

Secretary of State Hillary Rodham Clinton said Tuesday that the Syrian insurgents fighting to topple President Bashar Assad were making territorial gains that would eventually become safe havens for "further actions by the opposition."

Clinton also said the United States, which has pledged to provide nonlethal aid to the Syrian insurgency, would work closely with the rebels, an indication that the United States may take a more active role as they secure and hold swaths of Syrian territory. She also reiterated her call for Assad to start a political transition and vacate power.

Clinton's remarks, during a press appearance in Washington with the prime minister of Haiti, strongly suggested that the United States sees rebel momentum increasing in the 17-month-old uprising against Assad.

She spoke as new evidence of fracturing in his government emerged, with the reported defection of its ambassador to Cyprus, Lamia al-Hariri, reported by Arab satellite channels. If confirmed, al-Hariri's defection would make her the second ambassador to abandon Assad in the past two weeks. The first was his ambassador to Iraq.

There were conflicting accounts Tuesday of the fighting in Aleppo. A BBC correspondent, Ian Pannell, said in a Twitter post that Syrian forces had bombed Aleppo's eastern areas with fighter jets.

If confirmed, the use of warplanes would signify an escalation by the Syrian government in its effort to crush armed resistance. But Pannell did not say whether he saw the aircraft, and others in Aleppo did not immediately corroborate his assertion.

Unarmed U.N. monitors, whose operations were suspended more than a month ago because of the combat dangers, have observed Syrian helicopter gunships in fighting with Syrian rebels but not the use of warplanes.

The fighting in Aleppo, which entered its sixth day Tuesday, is considered significant because of the city's allegiance to Assad's ruling Alawite sect. Analysts have said Aleppo's loss would be a significant victory for the insurgents, who are mostly members of Syria's Sunni majority.

The developments came a day after the Syrian government warned foreign adversaries that it might use chemical weapons against them to thwart any possible outside intervention in the conflict.